1. Field of the Invention
The present invention relates to a transport system, and particularly to a transport system capable of conveying articles in a fabrication system.
2. Description of the Related Art
In a conventional fab, wafer processing steps are performed in discrete areas (bays). Wafers must be transported between bays (interbay) and within bays (intrabay). FIG. 1 is a schematic view showing a conventional fabrication system layout. A fabrication system 100 capable of fabricating semiconductor wafers comprises tool bays 11 and 15, and a plurality of transport systems.
Each tool bay comprises, in general, a number of tools for performing various wafer fabrication functions. For example, tool bays 11 and 15 comprise tools 11a to 11h, and tools 15a to 15h respectively. The transport system uses carriers for storing and transporting wafers. Within the fabrication system 100, an interbay transport system 19 transfers carriers between tool bays; intrabay transport systems 12 and 16 transport carriers within tool bays 11 and 15 respectively. Wafers cannot be transported between interbay transport systems and intrabay transport systems directly, but are transferred through stockers located at the end of bays. As shown in FIG. 1, stockers 13 and 17 serve tool bays 11 and 15 respectively. Stocker 13 has interbay load ports 139a, 139b and intrabay load ports 131a, 131b for transporting wafers to and from interbay transport system 19 and intrabay transport system 12 respectively. Stocker 17 has interbay load ports 179a, 179b and intrabay load ports 171a, 171b for transporting wafers to and from interbay transport system 19 and intrabay transport system 16 respectively.
Interbay transport usually originates and terminates in stockers, where the wafers wait for the availability of the correct tool for the next step. Generally, a fab with 25,000 wafer starts per month might perform an average of 200˜300 interbay moves per hour, with peak loading up to 500 moves per hour. An intrabay move involves removing wafers from the end-of-bay stocker and moving them to the first tool, and then from tool to tool within the bay, often requiring intermediate returns to the stocker to wait for the next tool's availability. Therefore, traffic density and intensity between the interbay transport system and the stocker increase with fabrication complexity. Conventionally, an end-of-bay stocker has one set of interbay and intrabay load ports for handling traffic between the end-of-bay stocker and the interbay and intrabay transport systems respectively. Each load port of the end-of-bay stocker serves as the only passage for wafers being transported to and from the stocker.
Such conventional fabrication system is prone to traffic jams at the load ports of stockers, and long delivery time and low transport capacity. Traffic jams in transport system result in not only long but also unpredictable delivery time, which causes difficulty in equipment scheduling and increases idle time in tools. As shown in FIG. 7, because of the frequently occurring traffic jams, a tool of the conventional fabrication system idles in wait for wafers available for processing. The idle time is presented as I in FIG. 7.
Hence, there is a need for a transport system better addressing the transport capacity problems arising from the existing technology.